It shall be assumed that when such a record carrier is used, all of the recording procedures as well as the structural and operative features of the record carrier material are sufficiently known. As may also be seen from DE-OS No. 25 20 581 (which corresponds to U.S. Pat. No. 4,054,922), there is also a problem that when such a record carrier is to be used, the dark recording line thereon, "light", is to be erased so that subsequently a dark recording line may be recorded on a light recording surface. Also, the specific extent of application of such recording process and similar recording processes is known. It comprises recording methods in which only the development of one value to be measured immediately before a certain happening is of interest, while older recordings are erased. This restriction allows the recordings to be stretched out so that there is a greater large scale recording of the measured quantity and so that a continuous use of new recording material is avoided.
A typical example of such a recording procedure is the recording of accident data, especially the recording of the speed of a vehicle over the distance last covered by the vehicle.
It is to be understood that a recording device suited for this recording procedure is subjected to very rough operating conditions and must therefore be designed correspondingly. This refers to the sensibility to outside magnetic fields, to shocks, as well as to temperature conditions. The special field of application of this device, due to its wide spread use, requires it to be absolutely reliable in functioning over a long period of time without any servicing. Also, it should be ready to function without any auxiliary forces and it should also be possible to install it in the vehicle without any special knowledge. This means that it must be easy to handle and that it should be independent of its place of installation in the vehicle. On the other hand, for such a device, the conditions of mass production have to be taken into consideration, which means the provision of a construction which is easy to mount, has a small number of parts and uses simple parts.
If, for a recording device of such kind, a record carrier of the above described kind is provided in a manner in which a "light" erasure of the recordings is desired, the means previously used for such erasure of the recordings cannot fulfill the above-mentioned conditions. According to prior suggestion, an optimal effect in erasing is only achieved when two magnetic fields, especially two magnetic alternating fields, become effective in the plane of the record carrier. The effective direction of the magnetic fields shall cross each other in the direction of the transport movement of such record carrier, which means that in each range of the record carrier to be erased, the fields shall be effective essentially at right angles to each other. Correspondingly designed erasure devices have relatively expensive electromagnets which are preferably magnetized by alternating current. The size of such erasure, resulting from the fact that they must grip the record carrier from its two sides, thereby makes them unsuited for this special field of application.
A generally satisfactory erasure may, for instance, also be achieved when a record carrier is moved in the field of an individual electromagnet in such manner that the range of the record carrier just leaving the effective range of the electromagnet is exclusively penetrated by field lines which are in the plane of the record carrier or which are only slightly inclined thereto. With this knowledge, as may be seen from DE-OS 25 30 092, it is possible to make erasing devices with an electromagnet which is much smaller and which may be moved transverse to the transport direction of the record carrier, the magnetic axis thereof being in a plane parallel to the record carrier and the movement of the electromagnet being essentially in the direction of the magnetic axis.
Such erasing devices with one or several movable electromagnets have certain advantages as compared with the already mentioned statically arranged erasing devices, particularly with respect to space and energy requirements. However, they require rather expensive driving means and suitable electrical connections in the form of sliding contacts which, as is well-known, are subject to wear and must be serviced. For the required purpose, which means for a recording device to be installed in a motor vehicle, the erasing devices as described in the above-mentioned DE-OS No. 25 30 092 are very unsuitable, especially since they function only when the necessary current supply is available.